Soul singer Claudia Lennear inspired both The Rolling Stones “Brown Sugar” and David Bowie’s “Lady Grinning Soul”.
If we are including biblical figures, then the devil is mentioned a hell of a lot.
Duke?
John Lennon must have a few, including Tom Paxton’s Crazy John, George Harrison’s All Those Years Ago & Elton John’s Empty Garden,
Rosanna Arquette is the subject of Toto’s “Rosanna,” and Peter Gabriel’s ["In Your Eyes] (don’t recall the exact title).
George W. Bush is highlighted in at least two songs:
Neil Young’s “Let’s Impeach the President”
Pearl Jam’s “Bush Leaguer” (which has the great line born on third, thinks he got a triple)
Billy the Kid and Jesse James both come up in a lot of country/folk/Americana songs.
Gloria?
River Phoenix is the subject of two songs that I can think of. One by Natalie Merchant and I think the other is by R.E. M. but I don’t remember, so if it’s no REM then it’s a similar band and the song is of mid- to late 90s vintage.
Also Firewater’s Hey Clown.
Just for the record, Pearl Jam didn’t originate that great line. Jim Hightower said it about George H.W. Bush at the 1988 Democratic convention, and he probably stole it from someone else. Link 
According to the wikipedia article, Marilyn Monroe has the following songs about her…
Elton John’s “Candle in the Wind” on Goodbye Yellow Brick Road
The Misfits’ “Who Killed Marilyn?” and “Die Monster Die”. The band takes its name from The Misfits
Los Prisioneros’ “¿Quién mató a Marilyn?”
Ray Anthony’s “My Marilyn”
Billy Joel’s “We Didn’t Start the Fire”
The Distillers’ “Gypsy Rose Lee”
Tori Amos’s “Father Lucifer”
Madonna’s “Vogue”
Suede’s “Heroine” on Dog Man Star
Jennifer Lopez’s “I’m Gonna be Alright”
Robbie Williams’s “The Actor”
The Kinks’ “Celluloid Heroes”
Def Leppard’s “Photograph”
Stereophonics’ “She Takes Her Clothes Off”
Blood Brothers has two songs called “Marylin Monroe”
The Spice Girls’ “The Lady is a Vamp”
Jay-Z’s “Hollywood”
Dan Bern’s “Marilyn”
Though I’m not sure I agree with all of the listings.
Aussie band TISM had a song called “I’m On The Drug That Killed River Phoenix”.
Elvis
Lotsa songs about Elvis, most of them reportedly pretty awful. A staple seems to be working song titles into the song itself, and singing about Elvis up there in Rock’n Roll Heaven.
David Allen Coe mentions Johnny Cash in at least 2 different songs, and I’m sure the Man in Black gets a nod in many others.
Marianne Faithfull inspired/was the subject of a few Rolling Stones songs.
& Carrie Ann by the Hollies was supposedly written about her, but the group was “too shy” to use her name.
I think some of the songs on the Monster album were about, at least in part, River Phoenix. The album is dedicated to him.
Speaking of R.E.M., they sure like Andy Kaufman: “Man on the Moon” and “The Great Beyond” are both about him.
Martin Luther King, Jr. has a few as well: U2’s “Pride (In the Name of Love)” and “MLK,” and Ben Harper’s “Like a King,” to name three.
Margaret Thatcher was an object of derision in tons of songs: “Celebrate (The Day After You),” by The Blow Monkeys, “Margaret on the Guillotine,” by Morrissey, and “Stand Down Margaret” by The Beat.
Peggy Sue
thanks… didn’t know that…
Perhaps surprisingly, Geronimo has inspired a whole slew of songs from '50’s instrumental rock to Modern Talking (I have a friend who collects them!)